At equilibrium with one atmosphere of air pressure, water will contain dissolved gases whose partial pressures will, quite naturally, sum to one atmosphere. Theoretically, neglecting gases such as argon and (for the moment) carbon dioxide, that one atmosphere of dissolved gas pressure will essentially consist of 0.21 atmospheres of oxygen pressure and 0.79 atmospheres of nitrogen pressure. When it comes to the concentration of each of these molecules in the water, however, the apparent 4:1 ratio of nitrogen to oxygen does not hold true. In actual fact, at 20° C., the water, at equilibrium, will contain about 15.3 ppm of nitrogen and 9.3 ppm of oxygen-less than a 2:1 ratio! This difference is due to Henry's Law and the ability of water to dissolve gases. The Henry's Law constant for nitrogen at 20° C. is approximately 79,000 (atm. per mol fraction) while that oxygen is approximately 44,000. Put another way, each ppm of dissolved nitrogen exerts a higher pressure than a dissolved ppm of oxygen. This difference, in areas such as aquaculture, can be exploited with the correct technology. Gas Infusion is such a technology.
It is known that the level of dissolved oxygen(DO) in the water can influence the health and growth rate of fish. Generally speaking, the higher the DO level in the water, the better is for the fish. (This is a general statement only. Obviously, there are/will be limitations.) One of the problems faced by the industry is finding an economical method of introducing higher levels of oxygen into the water. It is also known that the overall dissolved gas pressure can play a significant role in fish health and growth rate, etc. Just like humans who can undergo the ‘bends’ as a result of exposure to high (principally inert) gas pressures, fish can also be adversely affected if the total dissolved gas pressure is too high. Conventionally, the limitation here is that an increase in dissolved oxygen content would result in an increase in total dissolved gas pressure. Another factor that can affect health and growth rate, is the presence or build up in the water of noxious gases. Principle among these is carbon dioxide. Although present in the water in very low quantities as a result of atmospheric equilibrium, it can build up as a result of respiration. The major problem is that the pH of the water falls dramatically with even small increases in CO2 concentration.
Conventional gas/liquid mass transfer equipment or technology in the aquaculture industry seems focused only on achieving a minimum dissolved oxygen concentration in the water and not on what is really needed, reaching and maintaining an optimum water ‘atmosphere’.
In our prior U.S. patent application Ser. No. 09/307,423, filed 10 May 1999 (now U.S. Pat. No. 6,209,855, issued Apr. 3, 2001), the invention described involves the concept of gas infusion. The hydrophobic nature of a hydrophobic microporous hollow fibre membrane establishes a stable interface between an aqueous phase on one side of the fibre and a gas phase on the other. This interface remains stable so long as there does not exist a pressure differential between the phases in excess of the ‘breakthrough’ pressure required to ‘push’ the aqueous phase through the micropores, or the gas pressure exceeds the liquid pressure to such an extent as to bubble into the liquid phase. This stable interface can be used to transfer mass from one phase to the other. The disclosure of this patent is incorporated herein by reference.